There have been proposed several methods for determining various components in blood, particularly by analyzing the reaction products resulting from reaction between specific components in blood and specific enzymes. Measurement of blood glucose level is important to monitor the patient's status. It has been recommended that the patient measures his own blood glucose level every day by himself. In view of the recent increasing number of diabetics, there is a growing demand for a simple method and means of measuring blood glucose level with a minimum of pain.
Measurements of blood glucose level are usually accomplished by utilizing the enzymatic reaction to oxidize glucose. The enzymes for this purpose are glucose oxidase, glucose dehydrogenase, and the like. At present, colorimetric and electrical methods are used for determination of blood glucose level. The former method consists of attaching an indicator paper to a measurement device, introducing blood sample to the indicator paper which develops a color upon reaction with blood glucose and optically measuring the thus developed color. The latter method consists of measuring the amount of current flowing across electrodes in contact with the product resulting from enzymatic reaction.
What is essential for these methods is to collect blood samples for analysis. Before measurement, the patient has to collect his own blood. A common way to achieve this object is by sticking the patient's skin (that of the fingertip, for example) with a sticking device provided with a sticking needle and then squeezing out blood by pressing with fingers the surrounding of the point of sticking.
Unfortunately, the conventional optical measuring method using the indicator paper requires a comparatively large amount of blood and hence necessitates sticking the needle deep into the skin. This causes pain to the patient and intimidates many patients into giving up a self measurement of blood glucose level. This holds true for the electrical measuring method which also requires as much blood as the calorimetric method.
The conventional sensor used to electrically measure blood glucose level is usually composed of an insulating substrate or sheet, carbon or metal electrodes formed thereon by screen printing or the like, and a layer that absorbs blood dropped thereon. The layer contains an enzyme, electron acceptor, and electrolyte for pH adjustment. Many of the sensors are in the form of thin strip chip. In these sensors, the layer to absorb blood is formed at the distal end or side of the strip, so that it is given a blood sample directly. This structure presents difficulties in accurately placing a blood drop or tends to cause a failure.
If the sensor in strip form is integrated with a sticking needle, then the resulting device obviates the necessity of mounting the sticking needle and the sensor separately. This will simplify operation and improve usability. This idea, however, poses a problem of making it difficult to delicately adjust the sticking depth, which is the fatal cause for pain due to sticking.
Moreover, the device of integral type is complex in structure and tends to waste blood infiltrating into gaps between constituent members.